Ink jet printers are one type of apparatus for depositing droplets on a substrate. Ink jet printers typically include an ink path from an ink supply to a nozzle path. The nozzle path terminates in a nozzle opening from which ink drops are ejected. Ink drop ejection is controlled by pressurizing ink in the ink path with an actuator, which may be, for example, a piezoelectric deflector, a thermal bubble jet generator, or an electrostatically deflected element. A typical print assembly has an array of ink paths with corresponding nozzle openings and associated actuators. Drop ejection from each nozzle opening can be independently controlled. In a drop-on-demand print assembly, each actuator is fired to selectively eject a drop at a specific pixel location of an image as the print assembly and a printing substrate are moved relative to one another. In high performance print assemblies, the nozzle openings typically have a diameter of 50 micron or less, e.g. around 25 microns, are separated at a pitch of 100-300 nozzles/inch, have a resolution of 100 to 3000 dpi or more, and provide drops with a volume of about 1 to 70 picoliters (pl) or less. Drop ejection frequency is typically 10 kHz or more.
Hoisington et al. U.S. Pat. No. 5,265,315, the entire contents of which are hereby incorporated by reference, describes a print assembly that has a semiconductor body and a piezoelectric actuator. The body is made of silicon, which is etched to define ink chambers. Nozzle openings are defined by a separate nozzle plate, which is attached to the silicon body. The piezoelectric actuator has a layer of piezoelectric material, which changes geometry, or bends, in response to an applied voltage. The bending of the piezoelectric layer pressurizes ink in a pumping chamber located along the ink path. Piezoelectric ink-jet print assemblies are also described in Fishbeck et al. U.S. Pat. No. 4,825,227 and Hine U.S. Pat. No. 4,937,598, the entire contents of which are incorporated by reference.
Printing accuracy is influenced by a number of factors, including the size and velocity uniformity of drops ejected by the nozzles in the assemblies and among multiple assemblies in a printer. The drop size and drop velocity uniformity are in turn influenced by factors such as the dimensional uniformity of the ink paths, acoustic interference effects, contamination in the ink flow paths, and the actuation uniformity of the actuators.
In many ink jet systems, ink is supplied through a supply duct to a pumping chamber which communicates with a nozzle, and ink is ejected periodically from the nozzle by a rapid compression of the volume of the pumping chamber as a result of action by an electromechanical transducer, such as a piezoelectric element. The rapid compression is preceded and/or followed by a correspondingly rapid expansion of the chamber volume. During the expansion portion of the ink drop ejection cycle, the pressure of the ink in the pumping chamber is reduced significantly, increasing the tendency of any air dissolved in the ink within the chamber to grow bubbles on the surface of the chamber. Bubbles tend to grow in that manner, especially at nucleation sites in the chamber such as sharp corners, minute cracks or pits, or foreign particles deposited on the chamber surface, where gases can be retained. If the expansion/compression cycles occur at a sufficiently high frequency, the bubbles can increase in size from one cycle to the next, giving rise to rectified diffusion. The presence of gas bubbles within the pumping chamber prevents application of pressure to the ink in the desired manner to eject an ink drop of selected volume from the nozzle at a selected time, resulting in print quality degradation over time. Rectified diffusion can become more problematic in high quality ink jet systems because such systems tend to employ viscous inks that require higher pressures and frequencies to jet properly.
If the frequency of the pressure oscillations in the pumping chamber is relatively low, nucleation site bubbles are expanded within the pumping chamber, but re-dissolve before the next stroke as shown in FIG. 1. Bubble 20 is formed during an expansion stroke at time D. Later, during a compression stroke at time E, the bubble 22 is now smaller due to increased pressure and due to diffusion of the gas from the bubble back into the fluid of the pumping chamber. In this low frequency scenario, the bubble is dissolved by time F.
If the frequency of the pressure oscillations in the pumping chamber is relatively high, bubbles do not have time to re-dissolve during a compression cycle before being subjected to another expansion cycle. FIG. 2 illustrates how the bubble radius cycles to generally increasing radius over multiple pump cycles. FIGS. 3A-3C illustrate the effect of increasing bubble radius in the pumping chamber. Referring to FIGS. 2-3C, at time G, print element 30 fires droplet 32 during the compression stroke of the pumping chamber 34. Within the pumping chamber 34, with meniscus 33, bubble 36 has radius R36. Later, at time H, during a compression stroke, bubble 38 (not shown) has radius R38 that will grow even further during the next expansion stroke. Later at point I during an expansion stroke, bubble 40 has grown in size in print chamber 34 with meniscus 42. This process continues as before, producing bubble 44 (not shown) with radius R44 and bubble 46 (not shown) with radius R46. Finally, a significant bubble volume 48 is created in the pumping chamber. At this point, drop volume and velocity can be reduced or, in extreme cases, jetting can be prevented entirely because the energy that would go into jetting droplets goes instead to compressing the bubble.
Jetting at higher frequencies can be desirable because it increases throughput by allowing for higher line speeds. A primary limitation to operating frequency is the resonant frequency of the ink jet that is determined by the round trip time for a pressure wave in the pumping chamber. Therefore, making the pumping chamber smaller increases the natural frequency of the ink jet and allows higher operating frequencies. Making the nozzle diameter smaller also helps to operate at higher frequency, but this also requires smaller drop volumes. It also possible to jet at higher frequency by reducing the time over which the pressure is applied, but then higher pressures are needed. Typically, acoustic pressures range from about 2 atm below ambient on the expansion stroke and then to about 2-3 atmospheres above ambient during the compression stroke. Rectified diffusion can become more problematic at higher jetting frequencies.